The present invention relates to vehicle cranes of the kind which are provided with a hoisting arm arranged for swinging movement in the vertical direction relative to the crane stand or post by means of a hydraulic piston-and-cylinder unit. The hoisting arm comprises an inner arm section which is articulated to the post and an outer arm section which is articulated to the inner one and arranged for swinging motion by means of a hydraulic piston-and-cylinder unit and also is provided with a boom arranged for displacement substantially in the longitudinal direction thereof. It is already known to equip cranes of this type with a separate extension boom which is insertable into the ordinary extension boom in order to increase the working area of the crane. When the extension boom is extended to its extreme outer position, the total length of the hoisting arm of the crane does, however, become unproportionate compared with the main crane arm proper and difficulties arise in seizing or depositing goods in those areas of the vehicle loading platform that are positioned closest to the crane post. In addition, handling and maneouvring of loads suspended in the crane become difficult in narrow passages, such as factory halls wherein pillars and machinery sometimes block the way. Although the crane gains considerable reach (service range) and lifting height (elevation) its ordinary rocker arm joint will still, when the main arm is raised, remain at a comparatively limited level and in loading of goods in for instance building yards where walls and ceilings have a considerable height, it is sometimes difficult to lift the load above the walls and the ceiling elements because it is impossible to fold the rocker arm in over the crest of the wall. Attempts have been made to improve the working area of the crane by attaching to the ordinary, displaceable boom of the crane arm, a separate rocker arm which is then permanently secured thereto. The attachment arrangements for the separate rocker arm are, however, very space requiring and as a result the crane arm cannot be "parked" transversely across the vehicle but must be positioned in the longitudinal direction of the vehicle and thus encumbers the available loading space. However, it is required of cranes mounted on vehicles having a loading space thereon that it be possible to park the crane arm transversely across the vehicle in order to make the entire loading space free.